The corrosion requirements in technical applications, especially in the connector industry, are becoming more and more demanding. One example is the requested corrosion resistance where the efforts to standardise the technical requirements are partly not able to follow the market demands.
Gold plating is often used in electronics, to provide a corrosion-resistant electrically conductive layer on copper, typically in electrical connectors and printed circuit boards. Without using a barrier metal, the copper atoms tend to diffuse through the gold layer, causing tarnishing of its surface and formation of an oxide and/or sulfide layer. A layer of a suitable barrier metal like nickel is deposited on the copper substrate before the gold plating. The layer of nickel provides mechanical backing for the gold layer, improving its wear resistance. It also reduces the impact of pores present in the gold layer. Both the nickel and gold layers are usually deposited by electroplating or electroless plating.
To increase the corrosion, wear and heat resistance a layer comprising nickel and phosphorus can be used instead of pure nickel. With increasing phosphorus content the layer becomes less ductile and brittle which causes cracking and weakening of the parts. Furthermore the lower plating speed compared to nickel is another disadvantage because the velocity of the continuous plating line has to be reduced and the number of plating cells must be increased respectively.
In Götz, Heinisch and Leyendecker is described the optimisation of Ni/Ni—P/Au—Co layer combination to produce reliable connectors with reduced precious metals (W. Götz, T. Heinisch, K. Leyendecker, Galvanotechnik 9 (2003), 2130-2140). Herein the nickel-phosphorous layer is partly replaced by nickel, nickel-sulphamate in particular which has a higher plating rate and better ductility characteristics. For the qualification of the different layer combinations the IEC 61076-4-100/101/104 and the GR-1217-CORE standards have been used. Special connectors for telecommunication application have been used as test parts. As reference Ni/NiPd/AuCo plated connectors have been tested simultaneously. The Ni/NiP/Au plated connectors passed 10 days exposure to the 4-component mixed gas—according to IEC standard—and twice 125 insertion/withdrawal cycles. After 10 days storage for 21 days in damp heat (40° C., 93% RH) more than half of the test devices with Ni/NiP/Au failed whereas the Ni/NiPd/Au all passed.
The optimum layer thickness has been proven as Ni (1.5 μm), NiP (0.7 μm), Au (0.15 μm). The test criterion was the contact resistance.
The article gives no information about bending characteristics. The Ni—P layer thickness is with 0.5-1.0 μm still high. In addition there is no comment on the geometry of the connectors and consequently no hint whether these results are valid for different types of connectors with different geometries. The test criterion of the contact resistance gives reduced information for the contact area only but not for the adjacent areas.